docs(1.11.0): add manager-url setting documentation

[lexfrei]

Which issue(s) this PR fixes:

Issue longhorn/longhorn#10583

What this PR does / why we need it:

This PR adds documentation for the new manager-url setting introduced in Longhorn v1.11.0.

Changes:

• Added "Manager URL for External API Access" section in important-notes/_index.md explaining the feature, problem it solves, and configuration examples
• Added detailed "Manager URL" setting reference in settings.md with format requirements, validation rules, and use cases
• Cross-referenced both documentation pages for easy navigation

Why this is needed: The manager-url setting solves the issue where Longhorn Manager API returns internal cluster IPs (e.g., 10.42.x.x:9500) in actions and links fields when accessed through Ingress or Gateway API HTTPRoute. This happens when ingress controllers don't properly set X-Forwarded-* headers.

By configuring the external URL explicitly, users can ensure API responses contain correct external URLs accessible from outside the cluster.

Special notes for your reviewer:

• Documentation follows the same structure and style as existing settings
• Setting is placed in alphabetical order in settings.md TOC and content
• Cross-references between important-notes and settings pages are properly linked
• Version references use {{< current-version >}} shortcode as per project standards

Additional documentation or context

Related PRs:

• Code implementation: https://github.com/longhorn/longhorn-manager/pull/4245
• Helm chart update: https://github.com/longhorn/longhorn/pull/12050

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[derekbit]

We will do technical review soon. Thanks for the contribution.